1. Field of the Invention
The present invention relates to a coal combustor applicable to a coal gassification apparatus, a boiler or the like for business use and industrial use and a slag exhausting device therein.
2. Description of the Prior Art
At first, as one example of the prior art, a coal combustor in an entrained bed coal gassification furnace and a slag exhausting device provided at the bottom portion of the coal combustor will be described with reference to FIG. 5.
A slag exhausting device 11 is disposed at the center of the bottom portion of a coal combustor 12. Coal and char, combusted by burners 13 oriented in the circumferential direction of the combustor 12, first produce combustion gas as a result of the combustion, then produce combustible gas as a result of gassification. At the same time an ash content of the coal and char is converted to molten slag, which is centrifugally separated from the gas by a swirling stream 15 flowing from the burners, then adheres to a cylindrical wall surface 4 of the combustor 12 , flows down due to the gravity and accumulates at the bottom portion 16 of the combustor, and finally is exhausted via the slag exhausting device 11 into a slag chamber 17 and towards a slag hopper 18 disposed thereunder.
During this process, in order to facilitate the exhausting of the molten slag 14 through the slag exhausting device 11, it is necessary to maintain the molten slag 14 at a temperature as high as possible.
Although the molten slag 14 is held at a sufficiently high temperature and has a good fluidity (a low viscosity) at the bottom portion 16 of the combustor where the slag is subjected to strong radiation within the combustor 12, on the vertical surface of the slag exhausting device 11 the radiation is weak. Hence the temperature of the molten slag 14 decreases with a consequent loss in fluidity (its viscosity becomes high).
Therefore, as a contrivance for preventing the temperature of the molten slag in the slag exhausting device from decreasing, a bank 19 and a gate 20 around a slag hole as shown in FIG. 5 were adopted.
However, in the case in which a circular hole (slag hole) is open at the center (the central axis CA of the cylindrical combustor) of the bottom portion 16 of the combustor 12 in which a stream is flowing and in which the combustor is connected with a slag chamber 17 thereunder, due to a pressure distribution along the radial direction R within the combustor shown in FIG. 3, a descending flow flowing from the combustor 12 towards the slag chamber 17 is generated at the circumferential wall defining the hole (region A) while an ascending flow flowing from the slag chamber 17 towards the combustor 12 is generated at the central portion of the hole (region B) as shown in FIG. 4. The solid arrow in FIG. 4(b) shows the swirling direction of flow within the combustor whereas the dashed arrows show the direction of gas flow within a boundary layer at the bottom of the combustor.
Consequently, high-temperature gas within the combustor 12 descends along the lower surface of slag flowing to the slag hole, and the molten slag is heated by the high-temperature gas.
However, the prior art described above gives rise to the following problems.
(1) At the bottom portion of the combustor 12, molten slag flows out through the gate 20 only, and the bank 19 around the slag hole must be high. The vertical surface defining the slag hole is accordingly high. Hence, little heat radiates to the vertical surface from the combustion region and because the temperature of the slag decreases at the vertical surface, the fluidity of the slag decreases as well. PA0 (2) Furthermore, since increasing the height of the vertical surface defining the slag hole would further block the gas flow at the slag hole portion described above, a flow rate of high-temperature gas flowing from the combustor towards the slag chamber 17 along the circumferential wall portion would correspondingly decrease. Hence, such a measure would also result in the fluidity of the slag decreasing at the slag hole portion. PA0 (3) In the event that the decrease in fluidity of the slag is remarkable, slag would solidify (coagulate) in the slag hole portion, thereby blocking the slag hole and rendering the furnace inoperative. PA0 (i) The flare angle .theta. of the upper bank 2 of the slag exhausting device is 30.degree.-45.degree.; PA0 (ii) The radio ds/D of the inner diameter ds of the cylindrical lower portion 3 of the slag exhausting device to the inner diameter D of the combustor is 0.2-0.4; PA0 (iii) The ratio L/ds of the height L of the inner cylindrical vertical surface of the cylindrical lower portion 3 to the inner diameter ds of the cylindrical lower portion is 0.2-0.6; and PA0 (iv) The ratio H/D of a height H, taken between the upper surface of the bottom wall 5 and the top of the upper bank 2, to the inner diameter D of the combustor is 0.05-0.15. PA0 (1) Molten slag accumulated at the bottom of a combustor is forcibly collected in the slag exhausting device provided at the central axis of the cylindrical peripheral wall of the combustor under the influence of flows at a boundary layer at the bottom of the combustor and smoothly flows into a gate of the upper bank. PA0 (2) The flare angle of the upper bank is properly chosen so that a large an amount of heat radiates to the slag hole from the combustion chamber. Therefore, the amount of heat allowed to dissipate from the molten slag (the slag flowing through the gate and the slag flowing along the inner wall defining the slag hole) is suppressed, and accordingly, a lowering of the temperature of the slag is suppressed. PA0 (3) The height H, and the height L and diameter ds of the cylindrical lower portion are properly chosen so that a part of the high-temperature gas within the combustor flows into the slag hole jointly with the molten slag and flows out of the combustor. Therefore, the molten slag is heated and a lowering of the temperature of the slag can be suppressed.